Just curious, any opinions on the optimum diameter of a hollow grind? 10”, 8”, 6”? Seems like there would be an obvious answer, but I am not coming up with one by myself…
[OP]
Member
Just curious, any opinions on the optimum diameter of a hollow grind? 10”, 8”, 6”? Seems like there would be an obvious answer, but I am not coming up with one by myself…
Contributor
I've wondered this myself. I'd think that the smaller diameter, the better. As you move to larger and larger diameters, you approach a straight line which is what hollow grinding wants to avoid. A 6" should give you more honing before you need to re-grind.
Contributor
My thoughts are in agreement with Chuck on the nature of the hollow grind.
Another thought is the distance vs speed of the abrasive against the tool.
With a 6" wheel one revolution is a little less than 19", 8" is a bit more than 25" and 10" will be a bit more than 31". Multiply this by the rpm and it is easy to see how a bigger wheel can result in less time at the grinder.
There is also the price vs availability of the different size grinders and wheels to consider.
As Edward R. Murrow once said, "anyone who isn't confused really doesn't understand the situation."
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)
Contributor
I believe the optimum diameter is the one you currently have.
Richard
Contributor
I like larger. 6" leaves the edge too thin to suit me.
Contributor
Taking into account what's already been said, Optimum for what?Originally Posted by scott lipscomb
What are your criteria?
I think we would need a bit more info before answering with any substance.
Contributor
On chisels that require 30 to 35 degrees to support the cutting edge that may be a problem with hollow grind. Someone can do the math if they want.
Jim
Contributor
That is another interesting variable. The larger diameters will more abrasive pass over the steel per unit of time so you would expect that to produce more heat. Offsetting that is the greater wheel surface which would dissipate heat. I have no idea which effect is greater.
Contributor
Larger wheels usually turn at a slower rpm.
[OP]
Member
Per Edwards question above, I am talking about straight plane blades and chisels. Attached is a quick sketch of the section showing a 3" radius grind and a 5" radius grind on a 3/32" thick blade. Seems like a pretty insignificant difference.
hollow grind 2.pdf
Last edited by scott lipscomb; 12-08-2022 at 5:23 PM. Reason: improved diagram
Contributor
Makes more difference on a chisel that is a lot thicker? What are the edge angles for a chisel?
Contributor
Larger diameters make sense for chisels with a long face. Easy to position the hollow on a chisel. Any hollow is all you need. Plane edges make alignment with the wheel more difficult but again any hollow will do.
if the hollow alignment is too difficult then you probably don’t need one anyway.
You can do a lot with very little! You can do a little more with a lot!
[OP]
Member
Meant to point out, the hollow in the drawing in my previous post is 25 degrees, where I tend to grind my chisels and plane blades.
Contributor
I don't see a hollow, I see a flat bevel. Wrong file?
Contributor
I've had all three sizes. For about 7 or 8 years I used a Tormek with a 10" wheel. When I hone a blade with a hollow grind, I freehand directly on the hollow. The hollow then acts as a jig and balances the bevel on a waterstone.
The Tormek created a shallow hollow, and this would last about 3 sharpenings before the blade needed to be reground. Over time the 10" wheel became smaller, and eventually it reached 7". The deeper hollow lasted longer, understandably. However, this friable wheel was slow - owing to the slow speed it turned - and the changing size meant that setting were continuously being altered. I really like the blade holding system, however.
Eventually, I moved to an 8" half-speed bench grinder and two CBN wheels (80- and 180 grit). At the time I did this, nearly 8 years ago, no one I knew, outside of turners, were using CBN wheels. At least, no one had written about them for flat woodworking. At a result, I needed to gauge what grits to get. I chose 180 grit as the main wheel since this was the closest to the 220 grit of the Tormek.
The advantage of the 8" hollow is that it will last through about 10 sharpenings. In part, this is also due to the straight and clean edge at the bevel achieved by the combination of the ultra reliable CBN and the Tormek BGM-100 blade holder. It does not require much to create a bevel. This is a hollow grind with a micro bevel at about 10000 grit ...
So what about a 6" diameter? It will create a deeper hollow still, but also with a shallower entry. I have not heard of this being more fragile, and do see the 6" as a cheaper alternative to a 8". There are always pros and cons. The 8" wheel is expected to run cooler. Generally the wheels are wider, and the machines are more powerful and better built (than 6" machines).
Regards from Perth
Derek
Posting Permissions
Reply With Quote